One.relative_identifier(
  1 :: 2,
  key: 3 :: 4
) :: Two.relative_identifier(
  2 :: 1,
  key: 4 :: 3
)
One.relative_identifier(
  1 :: 2,
  key: 3 :: 4
)(
  4 :: 5,
  key: 6 :: 7
) :: Two.relative_identifier(
  2 :: 1,
  key: 4 :: 3
)(
  5 :: 4,
  key: 7 :: 6
)
One.relative_identifier key: 1 :: 2
One.relative_identifier unqualified 1 :: 2,
                                    key: 3 :: 4
One.relative_identifier 1 :: 2,
                        key: 3 :: 4